This invention relates to antennas for cellular telephone applications and, more particularly, to antennas suitable for communication from the stratosphere with earth-bound cellular users. Such users may, for example, be located in any of 96 cells arranged side-by-side in a plurality of annular bands of a circular coverage area, plus a circular central cell. Thus, with an antenna system at a lower stratosphere altitude of about 13 miles, thousands of cellular users within a circular area of the order of 400 miles in diameter may be served by a single cellular antenna system.
In current systems, in order to provide cellular service to users over a wide geographical area, a large portion of system cost relates to the need for multiple antenna sites. Due to tower height and earth curvature constraints, each antenna installation provides coverage over only a relatively small area of the earth's surface. The resulting requirement for a large number of antenna installations is further complicated by limits on availability of both suitable existing support structures for antennas and suitable sites for construction of new antenna towers. A compounding factor is a growing public aversion to additional antenna installations and towers.
As is known, the troposphere is that part of the earth's atmosphere in which temperature generally decreases with altitude, clouds form and convection currents are active. The troposphere generally extends from the surface to a height/altitude ranging from about 3.7 miles at the poles, to about 11 miles at the equator. In the stratosphere, which is located above the troposphere and continuing to a height of about 30 miles, temperature remains constant or increases slightly with altitude and relatively inactive atmospheric conditions exist. Surveillance aircraft, as well as weather balloon devices and other lighter-than-air craft, are known to operate at altitudes of 70,000 feet (13.3 miles).
Positioning an antenna system above the earth in the stratosphere for greater coverage is not practical with current types of antennas used for cellular applications. Such antennas, which are positioned and powered for coverage of cellular users located in a relatively limited geographical area, may typically provide from one to four antenna beams per azimuth sector. Even with four antenna beams per 90 degree azimuth sector, frequency allocation limitations and other constraints limit the total number of users which can simultaneously communicate via a single antenna system/site. Thus, if the coverage area of a single site were increased, operation would be subject to overload because the greater number of users in the expanded area could readily exceed the frequency allocation and operating constraints of a cellular site addressing a total of only 16 cells (four cells for each of four azimuth quadrants) for example.
As a result, even if usable with a lighter-than-air type vehicle able to be positioned relatively motionless in the lower stratosphere, the current types of typical cellular antennas would be unsuitable. This is particularly true because from stratospheric height the individual cell size (area of earth surface coverage) resulting from use of existing cellular antennas from such height would tend to encompass a greater number of users than could be serviced within frequency allocations and other system constraints. Existing antennas would be further unsuitable because of the vertical dimensioning of beams provided by such antennas and the absence of any provision for coverage in a central circular area which would lie directly below a stratosphere-located antenna.
Objects of the present invention are, therefore, to provide new and improved antenna systems for cellular use and, more particularly, to provide such antenna systems having one or more of the following characteristics and capabilities:
suitability for use from a stratosphere location; PA1 coverage of cells arranged in annular bands within a circular coverage area; PA1 provision of a number of simultaneous antenna beams (e.g., 97 beams) adequate to cover a large number of cells of reasonable individual area, from a stratosphere position (e.g., 13 miles altitude); PA1 provision of dual elevation beams to double the total of individual antenna beams and individual cells; PA1 enablement of frequency reuse patterns to permit multiple reuse (e.g., 16 times reuse) without unacceptable co-channel interference; PA1 enablement of frequency reuse patterns to provide thousands of channels (e.g., 5800 channels) for AMPS band A or B with one composite antenna system; and PA1 reduction of sidelobe level for a multi-beam antenna by inclusion of attenuators in outermost outputs of a beam forming network. PA1 (a) an array assembly with a plurality of laterally-spaced linear vertical arrays of radiating elements, PA1 (b) a plurality of first feed networks, each coupled to a vertical array to provide vertical beam coverage between a first pair of elevation angles representing the edges of a first annular band of the circular coverage area, PA1 (c) a first beam forming network having a plurality of output ports coupled to the first feed networks to produce a plurality of simultaneous side-by-side beams each providing coverage of a coverage cell located in the first annular band and within the first azimuth sector, PA1 (d) a plurality of second feed networks coupled to the first feed networks to provide, in conjunction with the first feed networks, vertical beam coverage between a second pair of elevation angles representing the edges of a second annular band of the circular coverage area, and PA1 (e) a second beam forming network having a plurality of output ports coupled to the second feed networks to produce a plurality of simultaneous side-by-side beams each providing coverage of a coverage cell located in the second annular band and within the first azimuth sector; and PA1 a plurality of additional 2-D antenna configurations, each of substantially identical description to the first 2-D antenna configuration, and each arranged to communicate with users within a different azimuth sector; PA1 the 2-D antenna configurations arranged to provide coverage of a total of at least 80 coverage cells within the two annular bands of the circular coverage area.